mprotect.2: Fix small bug in example program

The description of the example program says that it makes the
third page "read-only". Thus use PROT_READ instead of PROT_NONE.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/mprotect.2

@@ -207,7 +207,7 @@ main(int argc, char *argv[])
     printf("Start of region:        0x%lx\\n", (long) buffer);
 
     if (mprotect(buffer + pagesize * 2, pagesize,
-                PROT_NONE) == \-1)
+                PROT_READ) == \-1)
         handle_error("mprotect");
 
     for (p = buffer ; ; )